1. Technical Field
The present invention relates to a circuit for calculating a difference between a first input frequency and a second input frequency.
2. Related Art
In the related art, a frequency difference calculation circuit 20 is to calculate and output a difference between a first input frequency f1 and a second input frequency f2 as illustrated in FIG. 6. Such a frequency difference calculation circuit 20 has various applications. For example, the applications include an application of a phase-locked loop (PLL) for tracking a received signal and an application for adjusting an oscillation frequency of an oscillator having a low-precision to an oscillation frequency of an oscillator having a high-precision (for example, JP-A-2002-357652 and JP-A-2011-199481).
However, there has been a problem in a frequency difference operation circuit 20 of the related art. This problem occurs in a case where two input frequencies to be compared have a relationship of being an integer multiple, or a relationship that may be regarded as being a substantial integer multiple.
FIG. 7 is a view illustrating an example of experimental results obtained by calculating a difference between oscillation frequencies of two kinds of oscillator in a frequency difference calculation circuit 20 of the related art. The oscillation frequencies of the two kinds of the oscillator were adjusted to be approximately equal. In FIG. 7, the horizontal axis is a time axis (units are seconds) and the vertical axis is a frequency difference (units are ppm (parts per million)). At each calculation time, frequency differences obtained by calculating prior to the calculation time are cumulatively averaged and the cumulative average values plotted in a time series and are connected by a solid line. A true value of the frequency difference is “−1 ppm”.
As can be seen from the results of this experiment, the problem with the frequency difference calculation circuit 20 of the related art is in the time necessary for the outputs of the frequency differences to converge. If the calculation is performed for more than a certain period of time, the outputs of the frequency differences converge to a correct value, but a considerable time to convergence of the outputs of the frequency differences to the correct value is required. In the experiment in FIG. 7, the outputs of the frequency differences have not converged to a correct frequency difference even after the lapse of 100 seconds.